Objectives Appropriate indicators for iron (Fe) deficiency diagnosis was selected. Based on this, the main factors causing kiwifruit leaf chlorosis in central Shaanxi province were analyzed and verified using a fertilizer trial, which will provide a theoretical basis for preventing and controlling kiwifruit leaf chlorosis.

Methods In central Shaanxi province, three Fe-deficient kiwifruit orchards were identified through chlorotic leaf re-greening after Fe-EDDHA soil application. Sixty-four leaf samples with different chlorosis degrees were collected from the second and third fully expanded leaves at the top of the currently and vigorously vegetative shoots in these Fe-deficient orchards. The SPAD values of these leaf samples were correlated with five Fe-deficiency-diagnosis indicators. Moreover, five healthy and eighty chlorotic kiwifruit orchards in central Shaanxi province were selected to sample leaves with different chlorosis degrees (green, slightly chlorotic, and severely chlorotic), for determining twelve elements, active Fe, nitrate nitrogen (NO₃⁻-N) and SPAD values, and correlated these elements with SPAD values. Eighty-five soil samples were collected for analyzing physical and chemical properties. Additionally, a fertilizer trial in two chlorotic orchards was performed to test the correction of leaf and soil analyses.

Results In 64 Fe-deficient leaf samples, the correlation coefficient of leaf SPAD and phenanthroline Fe extracted from fresh samples was considerably higher than that of total Fe, 0.1 mol/L HCl Fe, and 1 mol/L HCl Fe, which were extracted from dry leaf samples, as well as 1 mol/L HCl Fe extracted from fresh leaf samples, indicating that phenanthroline-extractable Fe may be a better indicator for kiwifruit Fe deficiency diagnosis. No difference was found in leaf twelve elements, active Fe and NO₃⁻-N concentrations between green leaves of healthy and chlorotic kiwifruit orchards. The descending order of leaf SPAD and phenanthroline-extractable Fe concentration was: green leaf in chlorotic orchard > slightly chlorotic leaf > severely chlorotic leaf. The opposite was true for leaf NO₃⁻-N, phosphorus (P), potassium (K) and boron (B) concentration. The calcium (Ca), total Fe and manganese (Mn) concentration was lower in slightly and severely chlorotic leaves than in green leaf. Correlation analysis indicated that leaf SPAD positively correlated with leaf phenanthroline-extractable Fe, total Fe, Mn, and Ca, but negatively correlated with leaf K, P, NO₃⁻-N, total N, B and Cu. Moreover, random forest and path analyses indicated that the most closely related indicators to leaf chlorosis were phenanthroline-extractable Fe and K as well as NO₃⁻-N. Linear plus plateau function fitting model indicates that 5.084 mg/kg of phenanthroline-extractable Fe concentration was the critical concentration for diagnosing Fe deficiency in kiwifruit chlorotic leaves. When compared with the optimum ranges, the soil pH and HCO₃⁻ were higher but available K and NO₃⁻-N were suitable in chlorotic orchards, indicating that bicarbonate-induced Fe deficiency in soils may be the main reason of causing kiwifruit leaf chlorosis. Moreover, the soil cation exchange capacity was lower in chlorotic orchards than in healthy orchards, suggesting that leaf chlorosis is associated with the weak capacity of soil cation exchange. Application of EDDHA-Fe or vivianite Fe₃(PO₄)₂·8H₂O) increased leaf SPAD and phenanthroline-extractable Fe concentration, reduced leaf K and NO₃⁻-N concentrations, and thus correcting kiwifruit leaf yellowing induced by Fe deficiency.

Conclusions Phenanthroline-extractable Fe is the most suitable indicator for kiwifruit Fe deficiency diagnosis, and its critical concentration is 5.084 mg/kg. In central Shaanxi province, the leaf yellowing of kiwifruit orchards was mainly caused by bicarbonate-induced Fe deficiency in soils, and also related with the poor capacity of soil cation exchange. The accumulation of K and NO₃⁻-N in chlorotic leaves is the results of Fe deficiency. Therefore, more attention should be paid to the supplement of Fe fertilizers and the improvement of soil cation exchange capacity in chlorotic kiwifruit orchards.